At one time I asked my neurologist about TENS devices. She said they were not recommended for MSers, something about the electrical stimulation and the already impaired nervous system. TENS devices use a non-invasive technique to deliver electrical current in patterns determined by the needed treatment. Not so good.
However, that was years ago, and things may have changed.
I found an article in Revolution Health that explains the background and theory of using electricity for pain. Did you know the Egyptians used electric fish to treat pain in 2500 BC? This article says TENS remains controversial and better research is needed, but preliminary evidence is promising. The article continues with evidence for specific conditions and includes MS in the list of conditions with no evidence.
MSIF reported a study of TENS in MS that may lead to a change in that article. This study was designed to around spasticity, but found TENS may be useful for pain and muscle spasms in MSers.